1. Field of the Invention
The present invention relates to a flexible electroluminescence (EL) cell which is activated by an alternating electrical current (AC). More particularly, the present invention is directed to an easy-to-fabricate, flexible EL cell having non-adhesive properties to the plastic film substrate upon which it was formed as well as having a transparent conductive organic polymer layer contained therein.
2. Brief Description of Art
EL devices comprising a so-called “dispersion-type luminescent layer” which is formed by dispersing luminescent particles such as fluorescent substances in a matrix resin such as a polymer having a high dielectric constant are known from the following publications:
For example, JP-B-14878 discloses an EL device comprising a transparent substrate, a transparent electrode layer, an insulating layer consisting of a vinylidene fluoride base matrix resin, a luminescent layer comprising a vinylidene fluoride base matrix resin and fluorescent particles, the same insulating layer as above, and a rear electrode, which are laminated in this order.
JP-B-62-59879 discloses an EL device comprising a polyester film, an Indium Tin Oxide (ITO) electrode, a luminescent layer comprising a cyanoethylated ethylene-vinyl alcohol copolymer (a matrix resin) and fluorescent particles, and an aluminum foil (a rear electrode), which are laminated in this order.
U.S. Pat. No. 5,912,533 discloses an EL device whose front transparent electrode is made by using transparent conductive powder and transparent conductive binder. This EL device is made by a method comprising the steps: providing a substrate; forming a metal electrode layer on the substrate, wherein the metal electrode layer reflects light incident thereto; forming a dielectric layer comprising a mixture of dielectric powder and a binder on the metal electrode layer; forming a phosphor layer including phosphor powder and a binder on the dielectric layer; and forming a transparent electrode layer including transparent conductive powder and a transparent conductive binder on the phosphor layer using a spin coating or a screen printing process employed for liquid material.
FIG. 1 shows a cross-sectional view of a conventional EL device as described in U.S. Pat. No. 5,912,533.
The EL device shown in FIG. 1 comprises a plurality of layers including a substrate 11, a back electrode layer 10, a dielectric layer 4, a phosphor layer 6, a transparent electrode layer 1, and a polymer protection layer 5.
To fabricate the prior art EL device shown in FIG. 1, the back electrode layer 10 is first deposited on top of the substrate 11. Then, the dielectric layer 4 is formed on the electrode layer 10. The dielectric layer 4 may be made of a mixture of dielectric powder and binder for binding the dielectric powder, or a dielectric thin film. The dielectric powder may be BaTiO3, whose particle size is less than 3 micron. The binder, for example, may be made of a mixture of PVA (polyvinyl alcohol) type polymer and DMF (dimethylformamide) which works as a plasticizer. Next, the phosphor layer 6 is formed on the dielectric layer 4 by applying a mixture of phosphor powder 7 and binder 8 which binds the phosphor particles 7 together. The phosphor powder may be a II-VI group compound, e.g. ZnS. The particle size of the phosphor powder 7 ranges preferably from about 20 to 30 micron. It should be noted that the amount of the binder 8 required in the invention is less than that used in the conventional phosphor layer. As a result, an upper part of the phosphor particles 7 is exposed to be in contact with the transparent electrode layer 1 as shown in FIG. 1. It is possible to obtain three primary colors of light, i.e., red, green and blue, by mixing pertinent materials into the phosphor when forming the phosphor layer 6. For example, by adding Samarium (Sm) to ZnS, or by adding Cu, Mn and Cl to ZnS, red is obtained; by adding Terbium (Th) to ZnS, or by adding Copper (Cu) and Chlorine (Cl) to ZnS, green is obtained. By adding Thulium (Tm) to ZnS or by adding Cu and Cl to ZnS, blue is obtained. By making a layer with a mixture of materials related to the three colors, white light can be obtained. By using color filters on the white phosphor layer, it is possible to obtain various kinds of colored light. Subsequent to the formation of the phosphor layer 6, transparent electrode layer 1 is formed thereon by applying a mixture of ITO powder 2 and conductive binder 3. It is preferable to form the transparent electrode layer 1 by pressing the ITO powder and conductive binder 3 mixture with instant heating at the temperature of 100–200° C. so that the particles in the transparent electrode layer 1 are compactly arranged and the adhesion between the phosphor and transparent electrode layers is improved. As the transparent electrode layer 1 of the prior invention is made of material in a liquid state instead of the ITO thin film used in the conventional device. Moreover, as the phosphor powder 7 directly contacts the electrode layer 1, a strong electric field can be applied to the phosphor powder 7.
In this case the dielectric layer 4, phosphor layer 6, transparent electrode layer 1 are made of a material in a liquid state, i.e. a mixture of powder and binder, and can be easily fabricated by employing a spin coating or a screen printing method. During a spin coating process, a liquid material is poured on a substrate which is rotated so that the material is spread into a thin and uniform layer. During a screen printing process, a liquid material is put on a mesh made of silk or stainless steel and then rubbed with a soft plastic bar to allow it to pass through the mesh thereby forming a thin and uniform layer on a substrate.
It may be appreciated that the EL device shown in FIG. 1 has some disadvantageous effects including the low dielectric strength, high power consumption, low resolution capability by shaping or forming layers during lamination, high dielectric losses, major thickness of the device (0.3 mm), low efficiency, short a lifetime, poor flexibility.
U.S. Pat. No. 6,406,803 teaches making an EL device having a transparent substrate, a transparent conductive layer, a luminescent layer comprising luminescent particles and a matrix resin, and a rear electrode, wherein the luminescent layer has a transparent support layer comprising a matrix resin and the insulating layer comprising an insulating material, and a luminescent particle layer consisting essentially of particles which comprise luminescent particle and are embedded in both the support layer and the insulating layer.
U.S. Pat. No. 6,579,631 teaches making an EL device that includes a substrate, a lower electrode layer formed on the substrate, a light-emitting layer formed on the lower electrode layer, an upper electrode layer formed on the light-emitting layer, and a passivation layer formed on the upper electrode layer. The method for manufacturing an electroluminescence device includes the steps of forming a lower electrode layer on a substrate, forming a light-emitting layer on the lower electrode layer, forming an upper electrode layer on the light-emitting layer, and forming a passivation layer on the upper electrode.
These prior art EL devices have some disadvantages that include low dielectric strength, high power consumption, low resolution capability at the shaping or forming layer, high dielectric losses, major thickness of the device (0.3 mm), low efficiency, short operation life and poor flexibility. Many of these disadvantages are caused by the inclusion of an outer substrate layer in the EL device layer. It has now been found that EL devices not containing such an outer substrate layer do not have many of those disadvantages.